1. Field of the Invention
This invention relates to a water purifying process which is an ozone treatment for removing trace amounts of organic substances contained in water. The invention further relates to a method of treating water by injecting a chelate compound and then diffusing ozone therein.
2. Discussion of the Related Art
With the rapidly growing public interest in water, various water treatment processes have been examined. The purposes of these processes include obtaining safe and good-tasting water in the area of commercial drinking water and high clarity water from the recycling of sewage. In the case of commercial drinking water or service water, tap water sometimes generates musty odors caused by the eutrophication of rivers and lakes used as the source of the water supply. This problem is caused by so-called "musty odor substances" such as geosmin, dimethylisoborneol (to be referred to as "2-MIB" hereinafter) and the like which are produced by blue-green algae and generate musty odors when in extremely low minimum concentrations in the range of about 5 to 20 ng/l.
Although adsorption treatment with active carbon is carried out at most of the purification plants as a countermeasure to solve this problem, such a treatment is costly because of the need for increased amounts of active carbon and complex handling requirements. As an alternative to active carbon processes, advanced treatment methods such as ozone treatment, biological water treatment and the like, have been examined. In the case of sewage treatment, advanced treatment methods such as ozone treatments, membrane treatment and the like, have been examined both from a load-reduction viewpoint at the purification plant and an environmental viewpoint. Potential uses for purified sewer water include as wash water for use in cars and the like, and as scenery water, for example as moat water.
Under such circumstances, water treatment with ozone is regarded as an effective means for overcoming the aforementioned problems. The water treatment with ozone has the advantages of being stable and being easily controlled by electric means. This water treatment process, however, has problems of increased treatment cost due to the need for additional facilities and requiring sufficient space for the construction of a large reaction tank. As a consequence, when opting to use this process for water treatment, a purification plant will be confronted with serious problems of large construction costs and significant space requirements for the construction. Because of these problems, various modifications of the conventional ozone process have been proposed in order to materially reduce the size of the reaction tank, i.e., to improve efficiency of the reaction.
It is known that the oxidation reaction by ozone in water comprises the direct reaction of the ozone molecule and the indirect reactions of hydroxy radicals (OH.cndot.) and hydroperoxy radicals (HO.sub.2 .cndot.) which are formed by the self-decomposition of ozone. Since hydroxy radicals have stronger oxidation activity than ozone, efficiency of the water treatment with ozone can be improved by increasing quantity of the hydroxy radicals present. Such an improvement may be attained for example by:
(1) increasing the pH value of water to be treated, PA1 (2) adding H.sub.2 O.sub.2 to the water to be treated, PA1 (3) irradiating the water to be treated with UV rays, PA1 (4) irradiating the water to be treated with radiation, PA1 (5) irradiating the water to be treated with ultrasonic energy, or PA1 (6) employing a catalyst such as a metal ion or a metal oxide.
As an example of the above technique (6), a process has been disclosed in unexamined published Japanese patent application JP-A-59-186695 in which treatment efficiency of waste water that contains organic reducing materials is improved by the formation of chelate compounds effected by the addition of a metal ion such as of iron, manganese, cobalt or the like. According to the disclosed process, the electron density of the organic reducing materials is increased by converting the materials into chelate compounds to accelerate their reaction with ozone molecules and OH radicals which have high electrophilic reactivities.
The aforementioned techniques (1) to (6) for use in the acceleration of hydroxy radical (OH.cndot.) formation are effective in terms of improved reaction efficiency, but have the attendant disadvantage of large cost. The aforementioned metal ion addition process disclosed in JP-A-59-186695 has the added disadvantages of being a process that is effective only for the treatment of waste water which contains organic reducing materials and requires adjustment of the pH value of the waste water to 3 or below. In addition, when a large volume of water such as service water or sewage is treated by this catalytic process, it is necessary to increase the quantity of chemicals added to correspond to the waste water volume. This addition entails increased cost and the extension of facilities, thereby reducing the effectiveness of the process.